The Justice Department on Tuesday accused giant insurer UnitedHealth Group of overcharging the federal government by more than $1 billion through its Medicare Advantage plans.

In a 79-page lawsuit filed in Los Angeles, the Justice Department alleged that the insurer made patients appear sicker than they were in order to collect higher Medicare payments than it deserved. The government said it had “conservatively estimated” that the company “knowingly and improperly avoided repaying Medicare” for more than a billion dollars over the course of the decade-long scheme.

“To ensure that the program remains viable for all beneficiaries, the Justice Department remains tireless in its pursuit of Medicare fraud perpetrated by healthcare providers and insurers,” said acting U.S. Attorney Sandra R. Brown for the Central District of California, in a statement announcing the suit. “The primary goal of publicly funded healthcare programs like Medicare is to provide high-quality medical services to those in need — not to line the pockets of participants willing to abuse the system.”

Tuesday’s filing is the second time that the Justice Department has intervened to support a whistleblower suing UnitedHealth under the federal False Claims Act. Earlier this month, the government joined a similar case brought by California whistleblower James Swoben in 2009. Swoben, a medical data consultant, also alleges that UnitedHealth overbilled Medicare.

The case joined on Tuesday was first filed in 2011 by Benjamin Poehling, a former finance director for the UnitedHealth division that oversees Medicare Advantage Plans. Under the False Claims Act, private parties can sue on behalf of the federal government and receive a share of any money recovered.